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THHN generally comes in 7 strand, or more commonly 19 strand up to 4/0. From 250 MCM to 500 MCM, it's 37 strand. 600 MCM to 1000 MCM is 61 strand. I can't remember the larger sizes, but 127 strand rings a bell. The more strands it has, the easier it is to bend.

The number of strands have to do with the number of circles (strand cross sections) that fit inside a bigger circle (cable cross section), with minimum wasted space.
7 & 19 are magic numbers in this regard.

Jaime, if you had ever worked with 500 kcmil and larger, you would know the benefits of more strands. I love big jobs, but I don't look forward to pulling the services that's for sure! We once did a large industrial upgrade at a glass factory and had to lay alot of 750 kcmil in a cable tray. But it was spec'd in what was called "locomotive cable". I don't know how many strands it had, but it was flexible enough to jump rope with. Unfortunately my boss wouldn't buy it for every big job.

But for #6, sure, 4 strand would be fine for runs that didn't have a bunch of bends in them. Overall, the amount of copper is the same.

Jaime, if you had ever worked with 500 kcmil and larger, you would know the benefits of more strands. I love big jobs, but I don't look forward to pulling the services that's for sure! We once did a large industrial upgrade at a glass factory and had to lay alot of 750 kcmil in a cable tray. But it was spec'd in what was called "locomotive cable". I don't know how many strands it had, but it was flexible enough to jump rope with. Unfortunately my boss wouldn't buy it for every big job.

But for #6, sure, 4 strand would be fine for runs that didn't have a bunch of bends in them. Overall, the amount of copper is the same.

The last Locomovite cable I know it have over 150 strands so they are extra fexibale and also they are multi voltage rated 600v and IIRC 2KV as well.

I worked in a plant one time and we used locomotive cable for all the large DC motor conductors. From the drive to the motor. It really made it so much easier, especially the connections and the ability to get them back in the motor peckerhead.
It did cost big coin, but this one was on them not me.

I'm not positive how many strands were in the cable Lowes and home depot offered. But the 4 strand from the electrical supply was half price of lowes. My pulls for the hot tub are a bit rough, runs with 360 degrees of bends in them.
I am going to see if the electrical supply has something with a few more strands for a few bucks more.

Something I found that seems really weird, CH double breakers up to 60A are like $12-$14. The 70A double CH breakers are $45. Ouch.

I would not use very high strand count for house wiring. For an extension cord or appliance cord where flexibility really matters sure.

The thing is, once you get up to having very fine strands, oxygen in the sheath causes a loss of useful copper over the years. This is significant increase in resistance once you are talking about 150+ strands.